Nonwoven fabrics and numerous uses thereof are well known to those skilled in the textiles art. Such fabrics can be prepared by forming a web of continuous filament and/or staple fibers and bonding the fibers at points of fiber-to-fiber contact to provide a fabric of requisite strength. The term “bonded nonwoven fabric” is used herein to denote nonwoven fabrics wherein a major portion of the fiber-to-fiber bonding is adhesive bonding accomplished via incorporation of adhesives in the web to “glue” fibers together or autogenous bonding such as obtained by heating the web or by the use of liquid or gaseous bonding agents (usually in conjunction with heating) to render the fibers cohesive. In effecting such bonding, particularly autogenous bonding, the web may be subjected to mechanical compression to facilitate obtaining adequate bonding. Mechanical compression normally sets the loft or thickness of fabrics with similar basis weights. It is well known that thickness is increased by increasing the basis weight, or the mass per square area.
Spunbonded nonwoven fabrics formed of nylon, polyester, polypropylene, or other man-made polymers are widely used commercially for a number of purposes. Such fabrics exhibit excellent strength and permeability properties and accordingly are desirable for use in construction fabrics, filtration material, and furniture and bedding backing materials.
The fabrics are produced via the well-known spunbonding process in which molten polymer is extruded into filaments, and the filaments are attenuated and drawn pneumatically and deposited onto a collection surface to form a web. The filaments are bonded together to produce a strong, coherent fabric. Filament bonding is typically accomplished either thermally or chemically, i.e., autogenously. Thermal bonding is accomplished by compression of the web of filaments between the nip of a pair of cooperating heated calender rolls thereby setting the thickness. In autogenous bonding of nylon filaments, the web of filaments is transported to a chemical bonding station or “gashouse” which exposes the filaments to an activating agent (i.e., HCl) and water vapor. Water vapor enhances the penetration of the HCl into the filaments and causes them to become tacky and thus amenable to bonding. Upon leaving the bonding station, the web passes between rolls which compress and bond the web thereby setting the thickness. Adequate bonding is necessary to minimize fabric fuzzing (i.e., the presence of unbonded filaments) and to impart good strength properties to the fabric. Autogenous bonding has been used extensively in forming spunbonded nylon industrial fabrics.
Nonwoven fabrics which are strongly bonded overall (for example, by uniform compression of the entire web in the presence of heat and/or appropriate bonding agents) tend to be stiff and boardy and are frequently more similar to paper than to woven textile fabrics. In order to obtain softer nonwoven fabrics more closely simulating woven fabrics, nonwoven “point-bonded” fabrics have been prepared by processes which tend to limit bonding to spaced, discrete areas or points. This is accomplished by application or activation of an adhesive or bonding agent and/or application of heat and/or pressure at the points where bonding is desired. For example, the web to be bonded can be compressed between a pair of rolls or platens, at least one of which carries bosses or a land and groove, design sized and spaced to compress the web at the desired points. The compression device can be heated to effect thermal bonding of the web fibers or to activate a bonding agent applied to the web.
In the actual practice of preparing point-bonded fabrics, however, it is frequently difficult or even impossible to limit bonding to the desired points. In many processes, web areas between the desired bond points are subjected to sufficient heat, compression, activated bonding agent, or adhesive to effect “tack” bonding of fibers outside the desired bond points. Such tack bonding is believed to contribute significantly to undesired fabric stiffness.
It has been found that most point-bonded nonwoven fabrics, particularly those having a large number of tack bonds, and many overall bonded nonwoven fabrics can be significantly softened by subjecting the fabric to mechanical stress. For example, the fabric can be washed in conventional domestic washing machines, drawn under tension over a sharply angled surface such as a knife blade, stretched, twisted, crumpled, or subjected to various combinations of such treatments. Such treatments are believed to effect softening primarily by breaking weaker fiber-to-fiber bonds such as tack bonds which can be broken without breaking the point- or intentionally-bonded fibers. These methods are relatively effective but subject to certain practical problems. For example, drawing a nonwoven fabric over a knife blade with sufficient force to effect substantial softening frequently results in an undesirably high level of physical damage to the fabric. Washing of nonwoven fabrics generally yields good results, but is a batch operation not typically adaptable for use in continuous processes of the type employed commercially for production of nonwoven fabrics.
Another method for softening nonwoven fabrics is by impinging the fabric with a fluid jet. This is, however, an additional and potentially cumbersome production step, resulting in increased manufacturing costs.
It is apparent that a commercially practical process for a simpler, more cost-effective method for the softening of nonwoven fabrics while maintaining other advantageous physical properties such as strength and thickness would satisfy a long-felt need in the nonwoven textile art.
Thickness (loft) of nonwoven fabrics is normally determined by the basis weight. Increasing the basis weight adds cost due to the use of more raw materials. It is desirable to have increased thickness (loft) in some applications where these fabrics are used without increasing the basis weight.
Openness (air permeability) of nonwoven fabrics is also normally determined by the basis weight and method of bonding. In some applications, it is desirable to have a fabric with increased openness (air permeability) in some applications without increasing the basis weight.
Nonwoven fabrics are also used in a variety of coating applications. Coating materials will be captured and held more effectively onto a fabric that is more open. Fabrics that use less coating to effect the same desired results would be more cost effective. Fabrics with greater fiber surface area can also increase the effectiveness of the coating process.